Rechargeable lithium batteries can be constructed from a number of different materials including cobalt, nickel, manganese, phosphorous and iron. Each chemical combination has thermal limitations on both charging and discharging.
When discharging a battery, chemical reactions take place inside the battery that release electrical energy. In a rechargeable battery, these reactions can be reversed by application of electrical energy to the battery. The ability of the chemistry to be reversed without a loss of battery storage capacity is largely dependent on minimizing any unwanted chemical reactions that occur which bind the chemicals in the battery in irrecoverable ways. These unwanted reactions are called side reactions and they are the principal cause of battery degradation as the number of charge/discharge battery cycles increases.
In the case of low temperatures, the ability of the batteries to discharge is limited mainly by the speed of the chemical reactions that can occur. Generally, when a cold battery is overloaded, the voltage at the terminals will fall which naturally self-limits the battery in a safe manner.
In the case of low temperature charging, the speed at which the chemical reaction can be reversed is also slowed. Generally when a cold battery is charged too quickly the voltage at the terminals will rise causing the charging system to assume the battery is full, alternatively most charging systems will sense battery temperature and will reduce charging speed for a cold battery to ensure the battery is not charged too rapidly. Self heating of the battery may also help to improve charge acceptance for a cold battery.
In the case of high temperatures, the ability of the batteries to discharge is enhanced as the chemical reactions can occur quickly. Generally, when a hot battery is discharged, no damage will occur to the battery provided such discharge occurs at a rate that would normally be considered acceptable. The materials that makeup a lithium battery are somewhat dangerous if they are made very hot, for this reason the maximum discharge temperature for lithium batteries may be limited from a perspective of safety rather than damage.
In the case of high temperature charging, the battery will freely accept charge current at high temperatures. However, in this situation, the number of unwanted side reactions increases dramatically, causing irrecoverable damage to the battery. The approximate magnitude of these side reactions will double in magnitude for every 10 degree centigrade rise in temperature. It is therefore expected that a battery rated for 1000 charge/discharge cycles at a maximum temperature of 40 degrees centigrade would only achieve about 500 cycles at 50 degrees centigrade and about 125 cycles at 70 degrees centigrade. The majority of the cycle by cycle damage to the battery occurs during charging under these high temperature conditions.
The above limitations of temperature has generally resulted in lithium battery manufacturers providing a narrower acceptable temperature range for charging than for discharging. For example, lithium cobalt rechargeable batteries may be rated from 0 to +40 degrees centigrade for charging, but have a wider range of −20 to +70 degrees for discharging.
The ability to charge the batteries at lower temperatures is being addressed by charging systems that apply external heaters, or by reducing charge current into the batteries when they are very cold.
There exists a need for a method of charging that can reduce damage to batteries when charging at high temperatures.